Recently, a market for mobile communication devices, such as mobile phones, PHSs and PDAs, has been growing. Such mobile communication devices are moving towards slimness, lightness, low-power consumption, high resolution, and high brightness.
In particular, a device equipped with a touch panel or touchscreen as an input device has a structure in which a transparent conductive plastic film, for example, a polyethylene terephthalate (PET) base film having a conductive thin layer of an ITO formed on one side thereof, is stacked on conductive glass, a reinforcing material, or a decoration film through an adhesive film in consideration of lightness and breakage prevention.
Adhesives used for attaching a transparent conductive film in a touchscreen or touch panel need to have various physical properties, such as uneven surface leveling performance for relieving an uneven surface due to a decoration film, durability for controlling generation of curls or bubbles when exposed to severe conditions, for example, high temperature or high humidity, cuttability for preventing the adhesives from sticking out or being squashed when cut, and excellent adhesion to various substrates as well as optical characteristics, workability, and resistance to warping.
Particularly, a capacitive touch panel is designed to have a structure such that an adhesive film is directly attached to a conductive layer (for example, ITO). Thus, an adhesive layer employed for the capacitive touch panel needs to suppress a change in resistance of a transparent electrode formed of, for example, ITO.
A touchscreen or touch panel may be exposed to various environments during manufacture, storage, transport, and sale. Specifically, when the touchscreen or touch panel is exposed to severe conditions, for example, high temperature and high humidity, a substantial change in resistance can occur in an electrode of the screen or panel, causing serious damage or product defects.
One approach to solve such problems is addition of a tackifier or plasticizer to the adhesives. However, when a tackifier is added to the adhesives, modulus of the adhesives at low temperature increases, while durability at high temperature decreases. Also, when a plasticizer is added, modulus of the adhesives can decrease, whereas poor compatibility of the plasticizer with the adhesives can deteriorate reliability over time.